A
Civilization advance.
As tribes gave up their nomadic ways and settled into the first cities, they soon found they had a need for permanent buildings. Some citizens became experts in the techniques of masonry--cementing rocks and mud bricks into buildings and walls. With experience, they were able to build larger and more elaborate structures. Buildings grew, defensive walls became more imposing, and the masons developed aesthetic styles to complement the merely functional.
Prerequisites: none.
Allows for: Mathematics.